It is known that, due to stockage and transport necessities, the bituminous membranes in question are delivered in the form of rolls that are subsequently unwound by the users at the moment of their laying down.
In order to avoid undesirable glueing phenomena between two coils in a roll, due to the nature of the material (mainly consisting of bitumen), before the rolling step the membranes are coated with appropriate anti-sticking powders.
The application of said anti-sticking powders is carried out only on one side of the membrane (that is the one that during the laying step is turned outwardly) while the other side (the one in contact with the laying surface) is coated with a protective plastic film.
The anti-sticking powders applied on the membrane may be of various types. However the most preferred ones are mineral powders and, among them, talc powder which exhibits the best anti-sticking characteristics.
It is known that the currently-used methods for spreading powders on membranes can substantially be divided into dry methods and wet methods.
The application of powders using a dry method is carried out by means of appropriate vibrating nets that, being loaded with the powders, execute the spreading thereof on the underneath continuously running membrane which is transferred on rotating rollers.
This method however has some drawbacks. First of all it causes pollution in the work environment due to the dispersion of the powders in the air and furthermore it does not allow a homogeneous and optimum spreading of the powders themselves on the membrane. The pollution problem can be partially solved by the use of strong aspirators, which however results in a disadvantage from an economical point of view owing to the construction costs of the aspirators that evidently adversely affect the cost of the finished product.
The dry method used in order to obviate the above mentioned pollution problem provides the application of powders in an aqueous suspension. The application of the aqueous suspension on both sides of the membrane (obviously if the use of the protective plastic film is not provided) is carried out plunging the membrane itself into a suitable bath followed by a drying step which can be achieved by causing the membrane to move forward on entraining rollers for a given time.
Should the application of said suspension be needed only on one side of the membrane, said membrane is kept in a vertical position and caused to move in contact with and along a horizontal cylinder transverse thereto. The area disposed above the cylinder and defined by the cylinder itself and by the moving membrane is flooded with the aqueous suspension so that the aqueous suspension itself is distributed on the membrane surface in contact with the cylinder. As in the previous case, this step is followed by a drying step.
It is known that in both cases, however, it is not possible to reach the optimum spreading of the powders on the membrane. In fact, before and after the drying steps said powders tend to gather on preferential and rather broad areas threby creating other wide areas that, being unconcerned with the anti-sticking powders, give rise to the localized glueing of the roll coils.